The present invention relates to a semiconductor laser and a method for fabricating the same, and more particularly to a semiconductor laser ensuring a high output and a method for fabricating the same.
Semiconductor lasers are light sources for optical fiber communication and optical information processing. As compared to other kinds of lasers, the semiconductor lasers exhibit a high efficiency and a capability of a convenient and rapid modulation. In particular, the semiconductor laser have a miniature structure providing a convenience in use.
Depending on materials and compositions used, various semiconductor lasers may be used in a wide wavelength range of emitted light from the visible ray wavelength to the far infrared ray wavelength. Since the semiconductor lasers is ensured to be used for several tens of years, their application to technical fields is extending.
Such semiconductor lasers are classified into a GaAlAs-based laser oscillating at a short wavelength of 0.7 to 0.9 .mu.m and an InGaAsP-based laser oscillating at a long wavelength of 1.1 to 1.6 .mu.m. In terms of structure, a double hetero (DH) structure including n type and p type clad layers and an active layer interposed between the clad layers is known for semiconductor lasers.
Semiconductor lasers having the DH structure are also classified into one of the gain waveguide type and one of the refractive waveguide type. The gain waveguide semiconductor lasers having a stripe DH structure are lasers adapted to confine carriers and optical waves within a narrow active layer in a direction normal to layers grown and thereby guide light to a gain region of high carrier density.
The refractive waveguide semiconductor lasers are lasers adapted to confine optical waves in an active layer in a direction parallel to layers grown and thereby guide light. For these semiconductor lasers, a buried DH structure is mainly used.
The buried DH structure includes a pair of n type clad layers and an active layer interposed between the n type clad layers and has a shape that the active layer is surrounded by the n type clad layers. Since these lasers are constructed to achieve a wave guiding under a condition that the active layer is surrounded vertically and laterally by the clad layers of low refractive index, they are called refractive waveguide lasers.
In fabrication of semiconductor lasers having the DH structure, it is important to grow a thin film having the DH structure constituted by an active layer and n type and p type clad layers, on a substrate. This growth is called an epitaxy.
For such an epitaxy, various methods may be used which include a liquid phase epitaxy (LPE), a metal organic chemical vapor deposition (MOCVD) and a molecular beam epitaxy (MBE).
InGaAlP-based semiconductor lasers are fabricated by use of the MOCVD process because of the material characteristic thereof.
Where a semiconductor laser is fabricated by use of the MOCVD process, a thin film of a DH structure including an active layer and n type and p type clad layers, and a current shield layer are sequentially grown over a substrate using a primary MOCVD process. After etching the current shield layer, a cap layer is grown over the upper clad layer using a secondary MOCVD process.
For fabricating a semiconductor laser of the DH structure, therefore, growth of a thin film is achieved by using the MOCVD process at least two times.
FIG. 1 is a sectional view illustrating a gain waveguide semiconductor laser having an inner stripe structure fabricated by use of a MOCVD process.
As shown in FIG. 1, the semiconductor laser includes an n type GaAs substrate 11, an n type GaAs buffer layer 12 formed over the substrate 11, and a thin film formed over the buffer layer 12 and having a DH structure constituted by an n type InGaAlP clad layer 13, an InGaP active layer 14 and a p type InGaAlP clad layer 15. An n type GaAs current shield layer 16 is disposed on the p type clad layer 15 except for a current injection region A. For covering the current injection region A, a p type GaAs cap layer 17 is disposed over the exposed upper surfaces of the p type clad layer and the n type current shield layer 16.
The buffer layer 12, the n type clad layer 13, the active layer 14 and the p type clad layer 15 constituting the DH structure, and the current shield layer 16 are continuously grown over the n type substrate 11 in accordance with a primary MOCVD process. Meanwhile, the p type cap layer 17 is grown in accordance with a secondary MOCVD process.
Refractive waveguide semiconductor lasers have been also known. The refractive waveguide semiconductor lasers has a superior lateral mode characteristic over the gain waveguide semiconductor lasers. As such refractive waveguide semiconductor lasers, there are a semiconductor laser having a selective buried ridge (SBR) structure as disclosed in Toshiba Review, 45(11), 907 in 1990 and a semiconductor laser having hetero barrier blocking (HBB) structure.
FIGS. 2a to 2e are sectional views respectively illustrating a method for fabrication a refractive waveguide semiconductor laser. Now, this method will be described, in conjunction with FIGS. 2a to 2e.
In accordance with this method, first, an n type GaAs buffer layer 22, an n type InGaAlP clad layer 23, an InGaP active layer 24 and a p type InGaAlP clad layer 25, the latter three layers constituting a DH structure, are continuously grown over an n type GaAs substrate 21 in accordance with a primary MOCVD process, as shown in FIG. 2a.
In this case, the n type clad layer 23 and the active layer 24 and the p type clad layer 25 are formed to have thickness of about 1 .mu.m, about 0.1 .mu.m and about 0.9 .mu.m, respectively.
Over the p type clad layer 25, an insulating film 28 made of SiO.sub.2 or Si.sub.3 N.sub.4 is deposited, as shown in FIG. 2b . The insulating film 28 is then photo-etched so that it may be left on a portion of the p type clad layer 25 corresponding to a current injection region to be formed. The insulating film 28 has a stripe shape. By the photo-etching, the p type clad layer 25 is partially exposed.
Using the insulating film 28 as a mask, the exposed portion of p type clad layer 25 is etched to a predetermined depth, as shown in FIG. 2c. At this time, the etching of the p type clad layer 25 is carried out in a selective manner so as to form a stripe-ridge mesa portion 25-1 corresponding to the current injection region A. The remaining thickness of the p type clad layer 25 at the exposed portion thereof is about 0.25 .mu.m.
Thereafter, an n type GaAs current shield layer 26 is selectively grown over the resulting structure in accordance with a second MOCVD process, as shown in FIG. 2d. The growth of n type GaAs current shield layer 26 is hardly achieved in the current injection region due to the insulating film 28. As a result, the n type GaAs current shield layer 26 is formed only on the exposed p type clad layer 25.
The insulating film 28 is then removed, thereby forming the current injection region A. After removal of the insulating film 28, a p type GaAs cap layer 27 is grown over the resulting structure in accordance with a third MOCVD process. Thus, a semiconductor laser having the SBR structure is obtained.
Since the insulating layer 28 is used as a mask for the selective epitaxy of the current shield layer 26 in accordance with the method of FIGS. 2a to 2e, the semiconductor laser involves a problem of defects formed due to the insulating film 28 present in the current injection region A. As a result, the reliability of the semiconductor laser is degraded.
As the laser is activated, the bad effect caused by the insulating film reaches the interface between the cap layer and the clad layer and the active layer disposed beneath the clad layer.
In accordance with the above-mentioned method, the InGaAlP of the p type clad layer is exposed upon performing the epitaxy for forming the current shield layer 26 by use of the secondary MOCVD process and the epitaxy for forming the p type cap layer 27 by use the third MOCVD process. Due to such an exposure, oxidation of A1 and growth condition of GaAs for obtaining a good interface between the InGaAlP layer and the GaAs layer as the cap layer should be taken into consideration.
FIG. 3 is a sectional view illustrating a conventional semiconductor laser having an HBB structure.
As shown in FIG. 3, the semiconductor laser includes an n type GaAs buffer layer 32 formed over the substrate 31, an n type InGaAlP clad layer 33 formed over the n type buffer layer 32, an InGaP active layer 34 formed over the n type clad layer 33 and a p type InGaAlP clad layer 35 formed over the active layer 34. The n type clad layer 33, the active layer 34 and the p type clad layer 35 constitute together a DH structure. The p type clad layer 35 has a stripe-ridge mesa portion 35' in a current injection region A.
A p type InGaP current injection layer 36 is disposed only on the stripe-ridge mesa portion 35' of the p type clad layer 35. A p type GaAs cap layer 37 is disposed over the entire exposed upper surfaces of the p type clad layer 35 and the p type current injection layer 36 so as to cover the p type current injection layer 36.
In the semiconductor laser having the HBB structure, the energy band gap of the GaAs/InGaP/InGaAlP structure formed in the current injection region A and constituted by the cap layer 37, the p type current injection layer 36 and the p type clad layer 35 is stepwise exhibited, as compared to the GaAs/InGaAlP structure formed in a region other than the current injection region A and constituted by the cap layer 37 and the p type clad layer 35. As a result, the energy band gap in the current injection region A is varied smoothly so that most of carrier flows can be confined within the current injection region A.
Fabrication of the above-mentioned semiconductor laser having the HBB structure is carried out in a similar manner to that of the semiconductor laser having the SBR structure.
For fabrication of the semiconductor laser having the HBB structure, first, the n type GaAs buffer layer 32, the n type InGaAlP clad layer 35 and the p type InGaP layer 36, the latter two layers constituting the DH structure, are sequentially grown over the n type GaAs substrate 31 in accordance with a primary MOCVD process. Over the p type InGaP layer 36, an insulating film (not shown) is coated. Thereafter, the insulating film is removed at its portion other than the portion corresponding to the current injection region A, thereby partially exposing the p type InGaP layer 36 as the p type current injection layer.
Using the insulating film as a mask, the exposed portion of p type current injection layer 36 is etched, thereby partially exposing the p type clad layer 35. Thereafter, the exposed portion of p type clad layer 35 is etched to a predetermined depth so as to form the stripe-ridge mesa portion 35'. The portion of p type clad layer 35 subjected to the etching has a thickness of 0.25 .mu.m.
Thereafter, formation of the p type GaAs cap layer 37 on the exposed portion of p type clad layer 35 is carried out using a secondary MOCVD process. As a result, the p type current injection layer 36 is covered by the p type GaAs cap layer 37. Thus, the semiconductor laser having the HBB structure is obtained.
As the mask upon etching the p type clad layer 35, a photoresist film may be used in place of the insulating film.
Since the InGaAlP of the p type clad layer 35 is exposed upon performing the epitaxy for forming the p type GaAs cap layer 37 by use of the secondary MOCVD process, as in the gain waveguide semiconductor laser of FIG. 2, oxidation of Al and growth condition of GaAs for obtaining a good interface between the InGaAlP layer and the GaAs layer should be taken into consideration.